352 THE POPULAR SCIENCE MONTHLY. 



Does there exist in the 8uii (for it woukl evidently be useless to 

 seek elsevvliere) a, plicnomenon with :i similar period V If such is the 

 case, we sliall j^erhaps have laid our hand on a simple relation of cause 

 niid effect. At all events, it will be a valuable hint and a sort of first 

 indication of the track we ought to follow. Now, the spots on the 

 Sun observe a precisely analogous period. Every eleven years they 

 offer a strongly-marked, maximum of frequency, followed, after an 

 interval of several years, by a minimum, during which the Sun ap- 

 pears every day without a single accident or blemish on his brill- 

 iant surface. We are led, therefore, to investigate the case more 

 closely. 



More closely, in fact, we ought to look ; for the coincidence may 

 not be strictly exact. In that case, the present agreement of the two 

 phenomena would be purely accidental ; at the close of several periods 

 it would disappear, and we should have been the dupes of a mere illu- 

 sion. But M. Faye quotes a comparative table of the periods of the 

 solar spots and of terrestrial magnetism, drawn up by M. Wolf, of 

 Zurich, from which it appears that even the slight anomalies that oc- 

 cur, in resjject to the average period of one of these phenomena, are 

 faithfully reproduced by the other. This remarkable coincidence was 

 almost simultaneously pointed out by General Sabine, Monsieur R. 

 Wolf, of Zurich, and Monsieur Gautier, of Geneva. 



Thus the sjjots on the Sun those amplified whirlwinds which, by 

 digging hollows in his surface here and there, introduce into his brill- 

 iant shell masses (more or less considerable) of the cooler hydrogen 

 w^hich envelops it exercise on magnetism a daily action which is 

 perfectly sensible to us. The problem of these mysterious variations, 

 thus circumscribed, becomes henceforth more accessible. 



True, the problem is not solved by that solitary circumstance. 

 The advent of the spots determines two influences : first, they sensibly 

 reduce the extent of the active surface of the Sun, and consequently 

 of his radiations; secondly, they cause in the chromosphere, and far 

 above it, gigantic hydrogenous eruptions, whose effects we are unable 

 to appreciate. But that is precisely the point on which we have to 

 concentrate our means of investigation ; it is exactly there that Science 

 may hope to seize the word of the enigma. 



And, since there is an enigma, we are all the more strongly urged 

 to solve it, because the daily variations of the magnetic needle are 

 not alone in being affected by the variations of the photosphere. 

 The same mode of reasoning connects them with the appearance of 

 aurorse boreales. Here we are on familiar ground, discussing a phe- 

 nomenon visible to and admired by all. Already Arago had remarked 

 a sort of connection between the apparitions of the northern lights 

 and disturbances of the magnetic needle. The concomitance was 

 singular. And now we find that those aurorte present, exactly like 

 the variations of the magnetic dip, a period agreeing with that of the 



